The ability to transcend the surface appearance of human actions and make inferences about the intentions that motivate them represents a major developmental achievement that is crucial to social interaction as well as to social, cognitive, and linguistic development. Research has demonstrated that intention understanding originates in infancy, with key developmental shifts occurring at the end of the first year. However, important questions remain concerning the mechanisms underlying the development of intention understanding. The overall objective of the proposed research is to provide insight into how intention understanding develops. In particular, this proposal will reveal how motor achievements and experience in joint-attentive interactions with social partners contribute to intention understanding in infancy. The central hypothesis is that infants construct an understanding of intention (1) through motor experiences (means-end behaviors and self-locomotion) that highlight their own intentionality, and (2) through experiences in joint-attentive social interactions that occur with increasing frequency as infants begin to locomote independently. Two studies are proposed. In both studies, the key variable-intention understanding-is assessed through an experimental test of infants' ability to interpret and predict the intended outcome of an ongoing failed reaching action. Study 1 uses an age-held- constant design to examine associations between motor behavior, joint attention, and intention understanding in locomotor vs. prelocomotor 8.5-month-olds. The key question is whether locomotor infants are more likely to demonstrate intention understanding and engage in joint attention than age-matched prelocomotor infants. Study 2 uses a longitudinal design to examine motor behavior, joint attention, and intention understanding in the same infants both before and after the onset of self-locomotion. This longitudinal design allows examination of the factors that predict changes in intention understanding across the transition to self-locomotion. The proposed approach is innovative because it uses a combination of measures of online action processing, laboratory tasks of social-cognitive knowledge, as well as naturalistic measures of social interaction to investigate the emergence of a conceptual achievement at the intersection of three key domains: motor, social, and cognitive. The proposed research is significant because it is expected to expand understanding of the mechanisms underlying the development of intention understanding-a critical developmental achievement that is foundational for children's learning, language development, and social competence. Shedding light on the factors that contribute to the emergence of intention understanding will have broad implications both for understanding typical development and for understanding and intervening when social cognition does not follow typical developmental pathways (e.g., in the case of autism spectrum disorder). Ultimately, knowledge of how intention understanding develops has the potential to expand understanding of infant development and to help children achieve their full potential for healthy and productive lives.